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**7-3 Triangle Similarity: AA, SSS, and SAS Warm Up Lesson Presentation**

Lesson Quiz Holt McDougal Geometry Holt Geometry

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**Warm Up Solve each proportion. 1)**

2) If ∆QRS ~ ∆XYZ, identify the pairs of congruent angles and write 3 proportions using pairs of corresponding sides. z = ±10 Q X; R Y; S Z;

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Objectives Prove certain triangles are similar by using AA, SSS, and SAS. Use triangle similarity to solve problems.

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45 45

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**Example 1: Using the AA Similarity Postulate**

Explain why the triangles are similar and write a similarity statement. <A and <D are both 90 degrees, which means they are congruent. <ACB and <DCE are vertical angles. All vertical angles are congruent. Since two angles in both triangles are congruent, the triangles are similar by AA. Triangle ABC ~ triangle DEC

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**Explain why the triangles are similar and write a **

Check It Out! Example 1 Explain why the triangles are similar and write a similarity statement. <B and <E are both 90 degrees, which means they are congruent. In triangle ABC: If <A is 43 degrees, then <C is 47 degrees. <F = 42 degrees Therefore, <C is congruent to <F Since two angles in both triangles are congruent, the triangles are similar by AA. Triangle ABC ~ triangle DEF

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**Example 2A: Verifying Triangle Similarity**

Verify that the triangles are similar. ∆PQR and ∆STU Therefore ∆PQR ~ ∆STU by SSS ~.

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**Example 2B: Verifying Triangle Similarity**

Verify that the triangles are similar. ∆DEF and ∆HJK D H by the Definition of Congruent Angles. Therefore ∆DEF ~ ∆HJK by SAS ~.

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Check It Out! Example 2 Verify that ∆TXU ~ ∆VXW. TXU VXW by the Vertical Angles Theorem. Therefore ∆TXU ~ ∆VXW by SAS ~.

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**Example 3: Finding Lengths in Similar Triangles**

Explain why ∆ABE ~ ∆ACD, and then find CD. Step 1 Prove triangles are similar. A A by Reflexive Property of , and B C since they are both right angles. Therefore ∆ABE ~ ∆ACD by AA ~.

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Example 3 Continued Step 2 Find CD. x(9) = 5(3 + 9) 9x = 60

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Check It Out! Example 3 Explain why ∆RSV ~ ∆RTU and then find RT. Step 1 Prove triangles are similar. It is given that S T. R R by Reflexive Property of . Therefore ∆RSV ~ ∆RTU by AA ~.

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**Check It Out! Example 3 Continued**

Step 2 Find RT. RT(8) = 10(12) 8RT = 120 RT = 15

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Verify that ∆JKL ~ ∆JMN. ∆JKL ~ ∆JMN by SAS <MJN = <KJL (same angle in both triangles) Therefore ∆JKL ~ ∆JMN by SAS~.

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**Example 5: Engineering Application**

The photo shows a gable roof. AC || FG. ∆ABC ~ ∆FBG. Find BA to the nearest tenth of a foot. From p. 473, BF 4.6 ft. BA = BF + FA 23.3 ft Therefore, BA = 23.3 ft.

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Check It Out! Example 5 What if…? If AB = 4x, AC = 5x, and BF = 4, find FG. Corr. sides are proportional. Substitute given quantities. 4x(FG) = 4(5x) Cross Prod. Prop. FG = 5 Simplify.

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You learned in Chapter 2 that the Reflexive, Symmetric, and Transitive Properties of Equality have corresponding properties of congruence. These properties also hold true for similarity of triangles.

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